1. Field of the Invention
The present invention relates to a distance measuring apparatus of a so-called active type, in which a distance to an object is measured by projecting a signal and receiving a reflection signal thereof.
2. Description of the Related Art
In a distance measuring apparatus of a so-called active type conventionally well known in the camera arts or the like, a distance to a subject to be photographed is detected based on a principle of triangulation by projecting a signal light towards the subject and receiving a reflection light of the signal light.
Recently, a new distance measuring apparatus of the active type which has become popular uses a position semiconductor detector (PSD) as light receiving means for receiving the reflection light and can measure a distance without a moving portion.
The PSD outputs a signal according to a centroid position of a spot light received. Thus the PSD does not need uniformity of a projection image and allows a wide range of distance measurement. However, since the PSD has a resistance layer, resistance noises are unavoidable. A S/N ratio becomes worse for a far subject with a reduction in light the amount of reflection light, thereby decreasing the accuracy of distance measurement. Especially, the reduction in accuracy of distance measurement for a far subject has been a big problem with recent appearance of zoom cameras which permit far distance photography.
A solution to the above problem involves a method using a pair of bisectional silicon photo cells (SPC), in which a position of light reception of reflection light is detected by a ratio of outputs therefrom. However, if the SPC pair is used to cover the entire range of distance measurement from near to far, a sufficient base line length cannot be assured, thereby causing a disadvantage of reduction in the total accuracy of distance measurement.
Applicant proposed in U.S. Ser. No. 692,199 a distance measuring apparatus for measuring a distance to an object by projecting a signal and receiving a reflection signal thereof, which comprises first signal receiving means of a PSD for receiving a reflection signal of the signal reflected in a range from near to far distance, and second signal receiving means of a SPC pair for receiving a reflection signal of the signal reflected in a far distance, thereby increasing the accuracy of distance measurement on the far side and avoiding the reduction of total distance measurement accuracy.
The apparatus proposed has, however, a drawback of drops in distance measurement capability and in accuracy, because the reflection signal is received by both the first and the second signal receiving means, thereby decreasing an amount of signal to be received by each of the signal receiving means.